Modern wireless communication networks are typically placed under great demands to provide high data capacity within the constraints of the allocated signal frequency spectrum. In cellular wireless communication networks, capacity may be increased by re-using frequencies between cells, typically according to a predetermined frequency re-use pattern. A fixed wireless access system may comprise a base station, which may be referred to as an access point, typically mounted on an antenna tower, and a number of subscriber modules installed at customer premises. The area of coverage of an access point may be divided into sectors, each sector being used to transmit and receive with a respective cell. Capacity may be further increased within a cell by steering respective beams towards specific user equipments, which may be referred to as subscriber modules, to allow communication between the access point with improved gain and/or reduced interference reception in comparison with a beam covering a sector. The access point may be equipped with an antenna array and a beamformer for each sector, for forming beams for communication with each respective subscriber module. Capacity may be increased still further by employing Multi-User Multiple Input Multiple Output (MU-MIMO) beamforming, in which respective beams may be simultaneously directed at different subscriber modules selected for a MU-MIMO group, each beam carrying different data. The subscriber modules selected for membership of a MU-MIMO group have sufficient spatial separation that each beam may be directed at one member of the group, while a null is steered to the other members of the group, to avoid interference between groups.
A set of MU-MIMO beams may be formed by applying a beamforming weightset to a weighting matrix. The weighting matrix takes a number of input signals, and applies appropriate amplitude and phase weights to each input signal for transmission from each element of an antenna array, in order to form the respective MU-MIMO beam to transmit each input signal to the respective subscriber module. The beamforming weightset may be calculated on the basis of channel estimates, which relate to the amplitude and phase characteristic of the radio frequency propagation path through each transmit chain and each element of the antenna at the access point to each subscriber unit. The channel estimates may be calculated on the basis of sounding symbols having pre-determined phase and amplitude characteristics, which may be transmitted from the access point for reception at subscriber modules. The subscriber modules may then determine the channel estimates from the phase and amplitude characteristics of the received sounding symbols, and send response messages back to the access point indicating the determined channel estimates. However, the signaling overhead in sending the sounding symbols and responding by sending channel estimates can be heavy, and may limit capacity available for payload.
It is an object of the invention to mitigate the problems of the prior art.